Conventional robotic vacuum chuck end effectors transfer significant particulate contamination to the backside of the wafer at areas where the chuck contacts the backside of the wafer. Vacuum grips often cause backside contamination and/or damage to a wafer due to: (1) scratching the backside of the wafer—particularly from hard ceramic or anodized aluminum contacts; (2) particles from the grip pad that are worn off and left on bottom of wafer; and (3) particles that fall onto the grip pad and are pressed onto bottom of wafer. Vacuum chuck end effectors also have difficulty achieving a low leak rate vacuum seal if the wafer placed on the end effector is warped or imperfectly aligned with the end effector surfaces.
Edge support or edge grip end effectors minimize the amount of particulate contamination transferred to the backside of the wafer. Edge grip end effectors, however, present difficulties in accurate wafer pick up and transfer.
Gripping the perimeter edge of the wafer requires that structural features protrude from the upper surface of the end effector to a sufficient height such that the protrusions extend above the top surface of the wafer being gripped. This method also requires features to actuate the protrusions, extending and contracting them as required, which increases complexity and failure modes. Further, either the protrusion must be left as a remnant of a thicker piece of raw material than would otherwise be necessary, or the protrusions have to be attached to the end effector. The first method involves more raw material and machining. Also, these protrusions are not replaceable. The second method involves multiple parts and potentially complex permanent assembly methods, such as those discussed above. Edge gripping structures, however, can be expected to be relatively thick, and therefore difficult to maneuver through vertical stacking cassettes where the wafers are stored because the separation between wafers is on the order of only about 5–10 mm.
There are increasing uses for ultra-thin wafers—low profile chip designs, direct mount on boards, etc. These wafers are often less than 250 microns thick and are subjected to distortion and waviness. This presents challenges for handling thin substrates—which are conventionally not suitable for edge-grip technology end effectors.
One constraint with thin wafers includes ensuring that the vacuum source does not apply too much vacuum to the bottom surface of the wafer. A full 10 psi vacuum source, for example, will likely cause the wafer to warp. It is important, however, that the circular edge conform to the bottom surface of the wafer.
Accordingly, there is a need for a low profile vacuum end effector that includes workpiece supports that work in conjunction with distorted wafer surfaces. It is also desirable for the end effector to accommodate a wide variety of support materials and reliably retain the pads in place, while permitting damaged or worn supports to be readily replaced.